The invention relates to gas separations conducted by pressure swing adsorption, and in particular applications to oxygen or nitrogen separation from air and to hydrogen purification. A particular application is for oxygen enrichment to mobile fuel cell power plants, for which efficient and compact machinery will be required.
Gas separation by pressure swing adsorption (PSA) is achieved by coordinated pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorbent bed from a first end to a second end of the bed, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
A xe2x80x9clightxe2x80x9d product, depleted in the more readily adsorbed component and enriched in the less readily adsorbed component, is then delivered from the second end of the bed. A xe2x80x9cheavyxe2x80x9d product enriched in the more strongly adsorbed component is exhausted from the first end of the bed. The light product is usually the desired product to be purified by PSA, and the heavy product often a waste product, as in the important examples of oxygen separation over nitrogen-selective zeolite adsorbents and hydrogen purification. The heavy product is a desired product in the example of nitrogen separation over nitrogen-selective zeolite adsorbents. Typically, the feed is admitted to the first end of a bed and the second product delivered from the second end of the bed when the pressure in that bed is elevated to a higher working pressure, while the second product is exhausted from the first end of the bed at a lower working pressure which is the low pressure of the cycle.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. This conventional pressure swing adsorption process makes inefficient use of applied energy, because of irreversible expansion over the valves over large pressure differences while switching the adsorbent beds between higher and lower pressures.
The present invention is intended to enable high frequency operation of pressure swing and vacuum swing adsorption processes, with high energy efficiency and with compact machinery of low capital cost. The invention applies in particular to air separation.
The invention provides an apparatus for PSA separation of a gas mixture containing a more readily adsorbed component and a less readily adsorbed component, with the more readily adsorbed component being preferentially adsorbed from the feed gas mixture by an adsorbent material under increase of pressure, so as to separate from the gas mixture a heavy product gas enriched in the more readily adsorbed component, and a light product gas enriched in the less readily adsorbed component and depleted in the more readily adsorbed component. The apparatus includes centrifugal compression machinery cooperating with one or multiple PSA modules in parallel. Each PSA module comprises a plurality of adsorbers, with each adsorber having a flow path contacting adsorbent material between first and second ends of the flow path.
Each PSA module further has a first valve means cooperating with the adsorbers to admit feed gas to the first ends of the adsorbers, and to exhaust heavy product gas from the first ends of the adsorbers. Each PSA module also has a second valve means cooperating with the adsorbers to deliver light product gas from the second ends of the adsorbers, to withdraw light reflux gas from the second ends of the adsorbers, and to return light reflux gas to the second ends of the adsorbers. The term xe2x80x9clight refluxxe2x80x9d refers to withdrawal of light gas (enriched in the less readily adsorbed component) from the second ends of adsorbers via the second valve means, followed by pressure let-down and return of that light gas to other adsorbers at a lower pressure via the second valve means. The first and second valve means are operated so as to define the steps of a PSA cycle performed sequentially in each of the adsorbers, while controlling the timings of flow at specified total pressure levels between the adsorbers and the compression machinery.
The PSA process of the invention establishes the PSA cycle in each adsorber, within which the total working pressure in each adsorber is cycled between a higher pressure and a lower pressure of the PSA cycle. The PSA process also provides a plurality of intermediate pressures between the higher and lower pressure. The compression machinery of the apparatus in general includes a feed gas centrifugal compressor and a second product gas exhauster. The exhauster would be an expander (e.g. radial inflow turbine) when the lower pressure is at least atmospheric pressure. The exhauster would be a vacuum pump when the lower pressure is subatmospheric. A light reflux gas expander may also be provided for energy recovery from light reflux pressure let-down, and may for example be used to drive a light product compressor.
In the present invention, the feed compressor will typically supply feed gas, in several stages at discrete intermediate pressures for feed pressurization of the adsorbers as well as the higher pressure for light product production, to the first valve means. The exhauster will typically receive second product gas, in several stages at discrete intermediate pressures for countercurrent blowdown of the adsorbers as well as the lower pressure, from the first valve means. The light reflux expander may also perform pressure let-down on several separate light reflux stages, sequentially drawn from the second valve means at a set of discrete intermediate pressure levels, and after expansion returned to the second valve means at a lower set of discrete intermediate pressure levels. Heat exchangers may be provided to heat gas streams about to be expanded, for thermally boosted energy recovery.
In order for the flowing gas streams entering or exiting the compression machinery at each pressure level to be substantially uniform in pressure and velocity, each PSA module will preferably have a sufficiently large number of adsorbers for several adsorbers to be undergoing each step of the PSA cycle at any moment. During pressurization and blowdown steps, the several adsorbers passing through the step would be in sequentially phased converging approach to the nominal pressure level of each step by a throttling pressure equalization from the pressure level of the previous step experienced by the adsorbers. Flow is being provided to the adsorbers in a pressurization step or withdrawn in a blowdown step by the compression machinery at the nominal pressure level of that step. Hence flow and pressure pulsations seen by the compression machinery at each intermediate pressure level are minimal by averaging from the several adsorbers passing through the step, although each adsorber undergoes large cyclic changes of pressure and flow.
A preferred way to provide a large number of adsorbers in a mechanically simple PSA module is to install those adsorbers as angularly spaced elements in a rotor, whose opposed faces engaging across sealing faces with a ported stator sealing faces will provide the first and second valve means. By providing a sufficient number of ports with suitable angular spacing to accommodate each of the desired pressure levels (higher, lower and intermediate) in each of the first and second valve faces, a desired PSA cycle can be achieved. The present invention provides high surface area parallel passage adsorbers suitable for high frequency operation, these adsorbers being comprised of layered thin sheets supporting the adsorbent and with spacers between the sheets to establish flow channels, with the adsorbers installed to fill the volume of an annular cylindrical vessel, and with the adsorbers being angularly spaced about the axis of the annular vessel with the angular spacing of the adsorbers corresponding to the staggered phases with which the pressure swing adsorption cycle is conducted within those adsorbers. This aspect of the invention is usefully applicable without limitation to the case that the vessel is rotating to provide the valving function of the pressure swing adsorption process, and also to the case of a non-rotating vessel containing multiple adsorbers whose pressure swing cycle may be controlled by rotating multiport valves. In preferred embodiments, the cylindrical vessel is a rotor, whose rotation in engagement to first and second valve faces provides the valving function.
If a smaller number of adsorbers is used in each PSA module, surge absorber chambers will be needed to isolate each stage of the compression machinery from excessive pulsations of flow and pressure. With sufficiently large surge absorber chambers, flow and pressure pulsations seen by the compression machinery are again minimized.
The architecture of adsorbers has three main hierarchial levels to be addressed:
1) the micropores where selective adsorption takes place within the adsorbent media
2) the macropores providing access into the adsorbent media at approximately micron scale from the flow channels, and desirably with minimal mass transfer resistance so that departures from equilbrium between the micropores and the adjacent flow channels are always minimized,
3) the flow channels between bodies of adsorbent media, and along which a concentration gradient is established by the process.
In PSA gas separation using zeolite molecular sieve adsorbents, the conventional art has established a remarkable, precisely organized architecture at the atomic scale by which the micropores are defined by the zeolite crystal framework. The micropores are at approximately nanometer scale, and are organized up to the typical scale of zeolite crystallites of one or a few microns.
In conventional PSA technology, the zeolite crystallites are agglomerated into an amorphous macroporous structure to form adsorbent pellets or beads. The macropores are provided by the more or less random network of interconnecting cavities between the crystallites, allowing for space taken up by the binder. The resulting macropores will have a rather high tortuosity factor, multiplying the effective length of the macropores by a factor of typically three to increase mass transfer diffusional resistance correspondingly.
The adsorbent beads are typically formed at the scale of one or a few millimeters, and are loaded into the adsorber containment vessel to form a packed bed. The flow channels are provided by the voidage fraction between the beads, and typically have a length of the order of one meter. The random assembly of the packed bed, along with mixing events as the flow splits and recombines around the beads, results in axial dispersion which degrades the sharpness of the concentration wavefront established by the separation process. The packed bed also has inherently high pressure drop in the flow channels.
While prior art adsorbent beds based on zeolite molecular sieves are ideally organized at the micropore scale of the zeolite crystal lattice, their architecture is far from satisfactory at the scale of the macropores (bead architecture) and the flow channels (adsorber architecture). Packed beds of granular beads are subject to pressure drop and fluidization constraints which make it impracticable to operate with small diameter beads, much smaller than 1 millimeter diameter. The mass transfer macropore diffusional resistance of relatively large beads, further exacerbated by the macropore tortuosity factor, preclude efficient sustained operation at PSA cycle frequencies greater than approximately 10 cycles per minute.
An improved architecture of the adsorbent media bodies and the flow channels is one in which the adsorbent is supported in the form of xe2x80x9cadsorbent sheetsxe2x80x9d. The adsorbent sheets are thin sheets (either as the adsorbent with a composite reinforcement, or as an inert sheet or foil coated with the adsorbent), with the flow channels established by spacers as parallel channels between adjacent pairs of sheets. This xe2x80x9cadsorbent laminatexe2x80x9d configuration has much lower pressure drop than packed beds, and avoids the fluidization problem of packed beds. In experimental adsorbers tested to date, the adsorbent sheets are in the range of 100 to 175 microns thick. The channel width between adjacent adsorbent sheets of the experimental adsorbers has been in the range of 50% to 100% of the adsorbent sheet thickness.
Intermediate between the microscale of the zeolite crystallites and the macroscale of the laminate, the mesoscale architecture of the macropore network remains a challenge to be organized. The challenge is to improve on the highly tortuous macropore network provided by the amorphous structure of zeolite crystallites cemented together by conventional binders. Typical tortuosity factors in zeolite adsorbent pellets are in the order of 3 to 4. Straightening the macropores into a parallel bundle of straight pores orthogonal to the external surface of the adsorbent sheet would ideally result in a tortuosity factor of 1, greatly reducing macropore diffusional resistance which usually controls mass transfer. As the macropore diffusional time constant is proportional to the tortuosity factor and inversely proportional to the adsorbent characteristic dimension (pellet diameter or laminate adsorbent sheet thickness), a factor of 4 reduction in tortuosity is equivalent to a factor of 2 reduction in the characteristic dimension. Hence, for equal macropore mass transfer resistance at the same cycle frequency, the adsorbent characteristic dimension may be increased to reduce adsorbent flow channel surface area and consequently adsorbent manufacturing cost, and also to reduce pressure drop in the flow channels. Alternatively, the characteristic dimension may be held the same, and the reduced tortuosity and reduced macropore resistance may then be exploited to increase cycle frequency. This reduces the volume of the adsorbent, and again reduces the installed cost of the adsorbent.
Accordingly, an important aspect of the invention is alignment of macropore channels for improved high frequency PSA adsorbers. In this aspect of the invention, the adsorber provides contact between a flow channel and a wall of macroporous adsorbent material, with the macropore channels substantially rectilinear and orthogonal to the wall. The wall may be the surface of an adsorbent pellet, or more preferably the surface of an adsorbent sheet contacting a flow channel between parallel adjacent adsorbent sheets.
According to a further aspect of the invention there is provided an adsorber element for contacting an adsorbent material to a fluid mixture, the adsorber element being formed from layered sheets comprising the adsorbent material and a support material, with spacers between the sheets to establish flow channels in a flow direction parallel to the sheets and between adjacent pairs of sheets, the adsorber element having first and second ends defining a flow path in the flow direction through the adsorber element and along the flow channels established by the spacers.
According to another aspect of the invention there is provided an adsorbent structure suitable for use in a pressure swing apparatus, comprising at least one fluid flow channel; at least one wall defining said flow channel, said wall comprising a plurality of layered sheets of adsorbent material transverse to the flow channel, and wherein the sheets are spaced to define macropore channels between adjacent sheets, which macropore channels are substantially perpendicular to said flow channel.
Further objects and advantages of the invention will become apparent from the description of preferred embodiments of the invention below.